[whatwg/fetch] Proposal: `Viewport` header for early hints (Issue #1640)

See WICG proposal: https://discourse.wicg.io/t/proposal-viewport-http-header/3233/6

Currently using the `media` attribute in early hints makes them no-longer early, as they would have to wait for the document to be loaded and the first HTML chunk to be parsed to know if the media matches, as media matching relies on the viewport meta-tag.

It was proposed that we add a `Viewport` header, with the same string as the viewport meta tag, that hints the UA about the upcoming actual viewport, allowing us to resolve media-dependent early hints early.

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